Method and means for correcting sensitivity drift of amplifiers



N. R. GUNDERSON 2,570,665 METHOD AND MEANS FDR CDRRECTING sENsITIvITYDRIFT 0F AMPLIFIERS 5 SheetsSheet l Oct. 9, 1951 Filed oct. 18, 194895o; l SR... lus

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METHOD AND MEANS FOR- CORRECTING SENSITIVITY DRIFT oF AMPLIFIERS FiledOct. 18, 1948 i 3 Sheets-Sheet 2 i +550 vous 4e /40 Rf/Easmr 8 VOUS lll40 AMPL/F/ER z Maron [/0 VOLT$ AC.

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` METHOD AND MEANS FOR CORRECTING SENSITIVITY DRIFT OF' AMPLIFIERS FiledOct. 18, 1948 3 Sheets-Sheet 3 NORMA/v l?. Gu/VDERsO/V INVENTOR.

Tram/Ey Patented Oct. 9, 1951 TENT OFFICE METHOD AND MEANS FORCORRECTING SENSITIVITY DRIFT OF AMPLIFIERS Norman R. Gunderson,Pasadena, Calif. Application October 18, 1948, Serial N o. 55,033

19 Claims.

This invention relates generally to a method and system for compensatingfor changes in sensitivity in amplifier systems. It relates moreparticularly to an intermittently effective feedback system connected inan inverse or negative feedback sense, and primarily, though notnecessarily, intended for use in correcting or compensating forsensitivity drift in an electron multiplier amplifier with-or without aphototube input forming an electro-optical system.

The present invention is-primarily, though not necessarily, intended foruse in` automatically compensating for changes in phototube sensitivityin a scanning process, especially in a process in which photographicnegatives, positives or other images are scanned. The present inventionis described and illustrated in a form employing electron multiplierphototubes of the electrostatically focused type comprising aphotosensitive cathode and output anode and a plurality of electrostaticfocusing, secondary emission electrodes (commonly known as dynodes). TheR. C. A. 931 electron multiplier is exemplary of suc tubes, although theinvention is applicable when other various multiplier and even ordinaryphototubes are employed.

Electron, multiplier phototubes such as the R. C. A. 931 are subject toa very considerable degree of sensitivity drift primarily duringcontinued operation of the tube although there is some sensitivity driftdue to aging. The sensitivity drift due to continued operation isprobably caused by a number of cooperating factors such as temperaturechanges occurring in various elements of the phototube during continuedoperation thereof, such changes being either self caused or caused byvariations in ambient temperature, and an inherent instability arisingfrom the mode of operation of such an electron multiplier phototubewherein saturation current flows and relatively little space chargelimiting and stabilizing effect is employed. l Y

My invention provides an electron multiplier amplifier capable ofoperating in a stable manner (Cl. Z50-205) case the time factor limitsthe sensitivity drift to a relatively small value. A sensitivity driftcompensated electron multiplier phototube amplifier of the type I haveinvented will find eX- tremely wide application in the art, and indeedwill probably replace most ordinary phototubes heretofore used.

GenerallyA speaking, an illustrative preferred form of the presentinvention comprises an amplier system including a phototube and anamplifier effectively connected to the output of the phototube, andinverse feedback means responsive to amplified phototube output signalarranged to modify the light input to the phototube in a negativefeedback sense in accordance with the value of the amplified phototubeoutput signal whereby sensitivity drift of the system will be minimized.In the preferred form of the present invention means are provided forintermittently, alternately, effectively rendering the inverse feedbackadjustment means effectively responsive and nonresponsive to the ampliedphototube output signal whereby the light input to the phototube will beintermittently modified in a negative feedback sense in accordance withthe amplified phototube output signalduring the correspondingintermittent responsive periods.

The present invention may also include refer- -encing means forintermittently rendering the modulation of the input to the system(which may be electric input or light input) to be of a selectedreference value during the simultanei ous, intermittent periods when theinverse feedback means is effectivly responsive to the amplied phototubeoutput signal. The inverse feedback means may be arranged (wheneverduring a'responsive period the amplified phototube output signal differsfrom a selected standard value) to modify the light input to the photo-'tube in a manner tending to return the amplified .a plurality ofsecondary emission electrodes, and

the system input is the light input to the photosensitive cathode. Thereferencing means may also be arranged to intermittently and frequentlyapply a short-duration electric signal of a selected reference valueacross various or all of the secondary emission electrodes of theelectron multiplier phototube during the simultaneous, ini termittentperiods when the inverse feedback the amplified phototube output signalfrom the selected standard value and when the referencing means causesthe modulation of the light input to the phototube to be of a selectedreference value. In another preferred general form of the presentinvention, an electron multiplier phototube of the type just describedis employed, and the system input is an electric input 'signal appliedto the secondary emission electrodes of the electron multiplierphotot'ube.

The preferred form of the invention standardizes system input conditionsmomentarily during each intermittent peri-odrof .adjustment by negativefeedback means. During 4this intermittent period of adjustment thephototube current output regulates phototube light input in an inversefeedback manneruntil the phototube current output falls Within ia narrowpredetermined reference range. This virtually eliminates any change ineffective sensitivity of the photoelectric scanning unit. Thus theVpresent invention provides virtually complete Asensitivity back system.is employed it is extremely difficult 1^ to avoid the production ofoscillation or instability in the system. Most prior art negativefeedback amplifiers and servomechanisms inj tended for use with arapidly nuctuating input signal are therefore compromises so arranged asto avoid instability and Voscillation and yet have the least possible.error in sensitivity drift correction compatible with the instabilityand oscillations avoidance requirements.

Through the use of the present invention, instability and oscillationare avoided and yet during each negative feedback period the sensitivitydrift of the amplifier is virtually completely corrected without errorin a manner never before attainable.

Since the sensitivity drift in an amplifier, whether of the electronmultiplier phototube type or not, is a relatively slow thing, theintermittent correction thereof virtually without error is much moredesirable than the continuous relatively inaccurate sensitivity driftcorrection of an amplifier attainable through prior art systems andmethods.

With the above points in mind:

It is an object of the invention to disclose and provide a simple andefficient method of compensating for changes in response characteristicsof a phototube, such as an electron multiplier, at frequent intervalsduring the use of the circuit in which such tube is employed.

It is an object of the present invention to provide an improved negativefeedback system for virtually completely correcting sensitivity drift ofan amplifier system.

It is a further object of the present invention to provide an improvednegative feedback system for use with an amplier to intermittently applynegative feedback to the input to the amplifier in a manner tending tocorrect amplifier sensitivity drift intermittently.

It is a further object'of the present invention to provide an improvednegative feedback system for use with an amplifier to intermittentlyapply a selected reference input modulation to the amplifier andsimultaneously to apply negative feedback to the input to the amplifierin a manner tending to return the amplifier output to a selectedstandard Value having a fixed relationship to the vinput to theamplifier whereby to intermittently compensate for amplifier sensitivitydrift.

Other and allied objects will be apparent to those skilled in the artfrom a careful study of the illustrations, specification and appendedclaims.

To facilitate understanding reference will be made to the followingdrawings in which:

Fig. l is a Vdiagrammatic electrical schematic drawing of oneillustrative embodiment of the present invention applied to alogarithmic amplilier circuit.

Fig. 2 is a schematic diagram illustrating a modified feedbackcompensating system which may be used with a logarithmic amplifiersystem of the character shown in Fig. 1.

Fig. 3 is a diagrammatic electrical schematic drawing of a third formillustrating the application of the method to a system employing anantilogarithmic amplifier.

Fig. 4 is a schematic electrical diagram, together withran isometricview cfa scanning drum, showing means for periodically energizing theadjusting system of my invention. j

Fig. l illustrates one embodiment of the present invention employed inconjunction with a logarithmic amplifier system of the general type moreparticularly described in my co-pending application, Serial No.V702,172, filed October 9, 1946, now Patent No. 2,454,871, and alsodescribed =v and illustrated in my Patent No. 2,413,706 in conjunctionwith a mechanically driven scanning drum adapted to carry a photographicnegative or positive, or other image to be scanned by the photocathodeof the electron multiplier phototube forming part of the logarithmicamplier.

In the example illustrated in Fig. l, an electron multiplier amplifierprovided with m'eans for introducing an electronic input thereinto andsecondary emission controlling means, is shown. The specific form ofelectron multiplier amplifier shown in Fig. l is of the type wherein theelectronic input is provided by a photosensitve cathode 2 and whereinthe secondary emission controlling means comprises a plurality ofsecondary electron emission controlling velectrodes (commonly known asdynodes) indicated generally at 97. The electronmultiplier` phototube lalso includes an output anode 3. It should be noted that the pluralityofsecondary electron emission electrodes 91 are connected at spacedpoints along a resistor 4 through which acontrolled electric currentpasses, thus applying the proper poy tential to each of the secondaryemission electrodes 97 for controllably producing secondary electronemission from each of said electrodes and electrostatically focusing theemitted electrons upon the succeeding secondary emission electrode. Theoriginalsource of electrons is the photosensitve cathode which emitselectrons in response to light input thereto, which are thenelectrostatically focused upon the adjacent, succeeding secondaryemission electrode, this con- 75A tinuing until the last emissionelectrode focuses .the electrons emitted therefrom upon the output anode3.

The resistor 4 -is connected in series with a rectifier tube 9 andterminals 5 and 6 of a regulated, constant voltage power supply 1, thepurpose-of which will be more fully described hereinafter. Thephotocathode 2 of the electron multiplier tube is also connected to thelower end ofthe resistor 4 and is also connected to the negativeterminal of a power supply 38.

`It should be noted that the terminal 5 of the .power supply is at anegative potential with respect Ato ground and that the positiveterminal 8 of the power supply 1 is grounded, thus connect- 4ing theresistor 4 in series with an electron tube '26 through the cathode 21which is also grounded. The anodev 28 of the tube 26 is connected to thepositive terminal 29 of the power supply 38. Since the power supply 38,in the example illustrated, is of 1400 volts potential which isconsiderably greater than the potential supplied by power supply 1,normally speaking the anode of the rectifier tube 9 is negative withrespect to the cathode thereof, thus allowing no current to pass throughthe circuit .comprising rectifier tube 9, the resistor 4 and the powersupply 1. The only current which normally flows through the resistor 4is that which flows from the power supply 38 through the resistor 4 tothe terminal 5 of the power supply 1 to the positive terminal 8 thereof,which is grounded, through the cathode 21 of tube 26 to the anode 28 ofthe tube 26 and back to the positive terminal 29 of the power supply 38..Since this current flows through the tube 26,

Yit can be seen that the current .normally flowing through the-resistor4 (and therefore the potentials applied to the secondary emissioncontrolling. electrodes 91) is controlled by the bias of the grid 25 ofthe electron tube 26.

The effective over-all amplificaton'of `an electron multiplier tube ofthe type illustrated at I is controlled by the potentials applied to thesec- ;ondary emission controlling electrodes 91 and thereforeftheamplification of the electron multiplier phototube I is controlled bythe bias of the grid 25 of the electron tube 26. Potentials here givenare illustrative only and not limiting.

The output anode 3 of the electron multiplier `phototube I is connectedthrough a resistance I8 Ato the positive terminal 3| of a power supply28',

which is grounded at 32. The terminal 3| of power supply 28', in theexample, is at a 300 volt Vpositive potential with respect to ground.The

anode 3 is also connected to the grid II' of an electron tube I2, thecathode I3 of which is .grounded and the anode I4 of which is connectedthrough a resistance I5 to the terminal 3| of the power supply 28. Theanode I4 of electron tube 'I2 is also connected through a resistance I6to the grid I8 of an electron tube I9, the cathode 28 `of which isgrounded and the anode 2I of which is connected through a resistance 22to the positive terminal 3| of the power supply 28. 'Ihe lower end ofthe resistor I6 and the grid I8 are 'connected through a resistance I1to the lower terminal 21 of the power supply 28 which, in the exampleillustrated, is at a negative potential of inverse feedback means whichwill be more particularly described hereafter. The anode 2| of 4theelectron tube I9 is connected through a re- .Sistance 23 to the grid 25of the electron tube 26 Velectron 28|.

also connected through a resistance 285 to av 2I5 closes the switch 2|6.

6 and through a resistor 24, to the negative terminal` 21' of the powersupply 28. The lower end of resistor 22 is also connected to the anode200 of an electron tube 28 I, the cathode 282 of which is grounded andalso connected to a switch 283 arranged to selectively make contact withan electric contact 284 connected to the grid of the The grid and thecontact 284 are terminal 286 which is connected to the negative terminalof a low voltage power supply (such as V--15 volts) sufcient to biassaid tube beyond its cut-off point.

From the above description it can be seen that the output of theelectron multiplier phototube I is resistively coupled to the succeedingain-pliier stage including the electron tube I2 which is resistivelycoupled to the succeeding amplifier stage including tube I9 which isresistivelyrcoupled-to the succeeding amplifier stage including tube 26,and that the output of tube 26 (which corresponds to the output of themultiplier tube I), controls the current flow through the resister 4 andconsequently the potential across the dynodes of the electron multiplertube I, thus controlling the amplification constant thereof in aninverse feedback manner and tending to maintain the anode output currentof multiplier tube I virtually at a constant value, irrespective oflight input to the photocathode 2 of the multiplier phototube. Thevoltage applied across the bleeder resister 4 (which can be picked offYat the output terminal 49) has a virtually logarithmic relationship tothe light input to the photocathode 2. The operation of this logarithmicamplier is more fully described in the hereinabove mentioned co-pendingallowed patent application Serial No. 702,172, filed October 9, 1946,now Patent No. 2,454,871, and it will not be described in detail herein.

The input to the system in the example illustrated in Fig. 1 isunderstood to be light modulated by a photographic positive or negativeor other image by means of a scanning process. One form of scanningprocess which may be used for this purpose is illustrated in Fig. 4. Asthere shown, a photographic positive 281 is mounted on transparentcylinder 288. Light from light source 41 is focused on photographicpositive 281 by means of lens 289, and the light which passes throughphotographic positive 281 and transparent cylinder 288 falls onphototube I. Phototube I is connected to the amplifier 2I8 by means ofconductors 2| I, 2I2, and 2 I3.

Only so much of the scanning means has been shown as necessary to theunderstanding of the invention. The scanning unit consisting of lightsource 41, lens 289, and phototube I is a struc- Vtural unit which, asis well known in the art, is

driven in synchronisrn with the rotation of cylinder 288 so that thescanning unit slowly moves longitudinally of the cylinder.

A portion of the photographic positive 28T shown on Fig. 4 as shadedportion 2I4, is of uniform optical density for use in adjusting thesystem during the intermittent adjusting period. A cam 2 I5- is mountedon the same shaft as transparent cylinder 288 and operates the switchindicated generally at 2I6. During the intermittent adjusting periodwhen the scanning beam is scanning the area of uniform density 2I4, camConnection is then made between contacts 43 and 44 and between contacts283 and 284. Contacts 283 and 284 are connected to the amplifier 2 I8 bymeans of wires 2li-,and 21.8. Contacts 43 and 44 ofl switch 2.I6 lareconnected to motor 42 and the 110 volt A. C. power supply by means ofwires 21.9 and 22D..

The Operation of the system is such that during :the intermittentadjusting period the .scanning ,beamv scans the area of uniform density2I4 and ,atthe same time the two sections of switch 2I6 are closed. Thiscauses the intensity of light from the light source 4'! to be adjustedin an `inverse feedback manner by the rheostat 46 which; is in turncontrolled by motor 42, amplifier 2 I il' and phototube i until theanode current of phototube I falls within a narrow predetermined'Referenoing means are provided to intermittently apply a selectedreference Voltage or signal to the secondary emission electrodes 91. Inthe yexample illustrated this includes the electron `tube 2Q! which isconnected in parallel with the looi'itrolv of the flow of currentthrough resister 4. lthus applying a constant pre-selected referencesignal to the secondary emission electrodes 91, during thevintermittentl periods when the switch 203 is closed under the control ofthe scanning drum 233 carrying the image 213i; beingl scanned.

The scanning drum 2381' is also arranged to close the switch 43.simultaneously with the closure of switch 233, and theV image 23:?carried by the scanning drum or the scanning drum itself, is ,providedwith a strip 254 of constant optical 'density in a position such as tosimultaneously modulate the light originally emitted by lamp 4l' rby aselected reference density during the period -of time when the switches23 and 43 are closed, :whereby the modulation of the light input to thephotocathode 2 will be of a selected reference =value. intermittently atprecisely the same instant that a selected xed reference value poten--tial or signal is applied to the secondary' electrodes 9? andV atprecisely the same instant that the switch 43 is closed to render theinverse feedback means responsive to Variations in the amplied phototubeoutput current.

The inverse feedback means used during the intermittent adjusting periodincludes an electron tube 35, the grid 34 of which is connected througha junction i! to the output of electron tube I2. In this way the outputVoltage of tube I2 controls the current through tube 35 and through coil3S of a relay indicated. generally at 38. The relay coil 38 controls theposition of switch arm 39 positioned between two electric contacts 43and 4i. If the current through tube 35 and relay coil 33 is greater thanthe closing current of the relay, then connection will be made betweencontacts 39 and 4I and the motor will be caused to rotate in onedirection. If the `current through tube 35- and relaycoil 38 is lessthan the opening current of the relay then connection will be madebetween contacts 39` and 4l! and the motor will be caused to rotate inthe the relay armature.- During the K put signal.

vvoltage drop. in resistor lo v'slightly greaterY than the positivepotential above ground of kthe ter.- mirialrSI of DOWelsuDpl-y V28?. Inthe example Ygiven the voltage ofv terminalv 3| is +300 volts withrespect toV ground. If the current from anode 3 of phototubeI decreasesby a very small percentage from the mean value for correct adjustment,then the grid of tube I2 Will be made more Vpositive than the mean valuefor correct adjustment; VThe anode I4 of tube I2 and the grid 3 4 oftube 35 will in turn be made more negative than their respective meanvalues. The relay 38,',will be devenergized and connection will be; madebetween contacts 39 and 4D. This causes the motorv42to rotate in such adirection that'rheostat 46 increases the `current to scanning lamp 4l.The opposite action takes place if the current from anode -3 ofphototube; I is tenter-than the normal mean, value during the automaticadjusting. period. It Should be noted that the switch 43 which is closedonly during the intermittent checking periods under the control oi themechanically driven scanning drum 26S, ina-kes it' impossible for themotor 42 to be energized for rotation in either direction except duringthe intermittent checking periods. The shaft 45 ofthe motor 42`isarranged to .control the 'positionA of a variable rheostat 43 connectedto the scanning-lamp` 41' and through leads 48 to a suitableY source of`potential, in the vexample illustrated, 8. volts D. The motor 42 isconnected through leads 44 and switch 43 to a Silitable source ofpotential.V

Operation orthe above embodiment of the present invention can bevsummarized as follows. Periodically during the scanning of the image23? carried by theY driven scanning drum 28,fthe modulation of lightfrom the lamp 4l' is momentarily change by a selectedV xed referenceden,- sity, in the; example described by means oi a constant optical'density strip 2I'4 along'theedge of the image. At thesame time that thelight ismodiiied by the constant density optical strip (or by other.suitablezmeans), the switches 203 and 43 are momentarily closed. Theclosure of the switchy 203 interrupts the control of the multiplier bythe amplier and a xed reference value signal is applied to the secondaryelectron emission controlling means, in this case the dynodes 9T, thusstandardizing the amplification conditions or" the electron multiplierI. The simultaneous closure of switch 43 places motor 42 in condition`for energization in either direction under the controlfoff the relay38', which acts to energize` the motor in either of two directionswhenever the anode current ofphototube I- deviates from a selectedstandard value. The energization of the motor 42 takes place in adirection such as to rotate the variable rheostat 46 in a manner wherebytocontrol the electric current ow through the lamp 47 in a negativefeedback sense., Thus the system provides for a standard inputmodulation during the checking periods, a

standard voltage applied Vto the secondary. emissiony controlling meansduringl theV checking iperiod, and provides for modif-ying the lightinput tothe phototube in a negative'feedback sense in response-todeviation' of the outputy signal from a standard' value during theychecking period, until such standard Value is reached by the out- Thusvduring each checking period, the sensitivity drift of vthe electronmultiplier amplifier is Virtually completely compensated,

.The checking periods may. talee place atv the-'fend logarithmic ampliersystem shown in Fig. 1 at the junction 50 in lieu of the inversefeedback means of Fig. 1 shown below junction 50. The lead from thejunction 50 may be connected to the grid of an electron tube 52, thecathode 53 of which is grounded and the anode 54 of which isconnected'through a resistance 55 to a terminal 56 adapted to beconnected to suitable source of positive potential. The anode 54 of theelectron tube 52 is also connected through a resistance 51 to one end ofa switch 60 and also through a resistance 58 to a terminal 59'adapted tobe connected to a suitable source of negative potential.

The switch 60 is arranged to close a circuit to grid 6| of an electrontube 62, the anode 64 of which is connected `to a terminal 65 adapted tobe connected to a suitable source of positive potential and the cathode66 of which is connected through a resistance 61 to a terminal 68adapted to be connected to a suitable source of negative potential.

to ground. It should be noted that the switch 60 corresponds in functionto the switch 43 shown in Fig. 1 and is arranged to be intermittentlyclosed by the scanning drum 208 (or other timing means) in the samemanner and at the same time as switch 203. The cathode 66 of theelectron tube 62 is connected in parallel through resistances 69 and 18to grids 10 and 19 of electron tubes 1| and 80. which form the outputstage of an oscillator, the cathodes of which are connected through aresister 201 to ground and the anodes 88 and 89 of which are connectedto opposite ends of a primary coil 90 of an output transformer, thecenter tap of which is connected lto aterminal 9| adapted to beconnected to a suitable source of positive potential. The secondarywinding 92 of the output transformer is connected to the scanning lamo93. which corresponds tothe scanning lamp 41 of Fig. 1.

An oscillation generator indicated generally at 81 includes electrontubes 11 and 86 4coupled to the tubes 1| and'fl through leads connectingthe anodes 16 and 85 through resistances 15 and 84 to points 13 and 82connected through capacitors 12 and 8| to the grids 10 and 19` of thetubes 1| and .80. The points 13 and 82 are also connected throughresistances 14 and 83 to the terminal 9| and tbe source of positivepotential.

VDeviation of thevphototube output signal is fed through the switch 60to the grid 6| offelectron tube 62 modifying the bias thereof andcurrent therethrough, thus modifying the grids and 19 -of tubes 1| and80 and modifying the output oscillations of the tubes 1| and 80 whichare fed by th'e oscillation generator 81. The transformer secondarywinding 92 feeds oscillations to the 'lamp 93 which have been modied asherein- The grid 6| of the electron vtube 62 is also connected through acapacitor 63 Anegative feedback sense.

10 current to the grid 6| and preventing capacitor 63 from becomingcharged or discharged between adjusting periods.

The embodiment of the present invention illustrated in Fig. 3 is amodication intended for use with an antilogarithmic amplifier system ofthe type more specifically described in my co-pending application SerialNo. 702,172, now Patent No. 2,454,871. In this modification of theinvention, the electron multiplier phototube 94 is provided with aphotocathode 95, anode 96, and a plurality of secondary emissionelectrodes 91, connected across a resistor 98, the upper terminal 99 ofwhich is connected to a source of negative potential and the lowerterminal |00 of 'which is arranged to be connected to receive the systeminput signal. The anode 96 of multiplier tube 94 is connected through aresistance |0| to terminal |02 of a power supply |03 which,in theexample shown, Vis at approximately volts positive potential withrespect to ground. The anode 9S is also connected to grid |04 of anelectron tube |05, the cathode |06 of which is connected through aresistance |01 to ground. The anode of tube |05 is` connected to lights||2 and ||3 which are connected to terminal 4 of the power supply |03,which may be at approximately 350 Volts positive potential with respectto ground. Light source 2 (as modulated by vane |40) is directed uponthe cathode of tube 94, whereas light source ||3 is the scanning light.

Cathode |06 of the output tube |05, is coupled through resistance |08 tothe grid |09 of electron tube ||0, the cathode 5 of which is groundedand the anode ||1 of which is connected through a resistance |4| to thepositive terminal |4 of the power supply |03. Grid |09 of'tube ||0 isconnected through a resistance ||9 to the negative terminal |25 of apower supply |26. Said negative terminal |25 in the example illustrated,is at approximately 350 volts negative potential with respect to ground.Anode ||1 of tube ||0 is connected through the resister |20 to grid |22of an electron tube |50, the cathode ||6 of which is grounded and theanode ||8 of which is connected through a resistance |42 to the positiveterminal ||4 of the power supply |03. Grid, |22 of tube |50 is alsoconnected through a resistance |2| to the negative terminal |25 of thepower supply |26. Anode ||8 of tube |50 is also connected through aresistance |23 to the grid |21 of an electron tube |5|, the cathode |28of which is grounded and the anode |29 of which is connected through arelay coil |30 to the positive terminal ||4 of the power supply |03.VThe grid |21 of tube |5| is also connected through a resistance |24 tothe negative terminal |25 of the power supply |26.

A control switch having amovable element |32 positioned between opposedelectric contacts |33 and |34 is arranged to be actuated by the relaycoil |30, thus closing either of two motor circuits of thereversiblemotor |35 in a manner similar to that previously described in connectionwith motor 42 in Fig. 1, thus rotating the motor shaft |39 in either oftwo directions', positioning a vane or light valve |40 of suitableconfiguration between the light source and thev photocathode ofmultiplier 94. Whenever Vswitch |36 is closed during a'checking period,vane |40 will-modify the light impinging on the photocathode 95 in aSwitch |36 may be actuated in any suitable manner andresembles switch 43in characteristics. v

It should be noted,lthat in this case the input 11 to the-system is anelectric input signal applied to the-secondary-emission controllingmeans and that the output from the system corresponds to the phototubeoutput signal and can be taken `from-the system at any suitable point.It may comprise the light emitted from either or both ofthe light`sources lf2-U3 or it may comprise the current iiowing therethrough orthe amplifled-signal correspondingthereto. It will be seen that inY.this case the relationship of output to input is virtuallyantilogarithmic, Whereas the relationshipbetween-system output andsvstem input in the logarithmic circuit employed in conjunction with theapparatus of the present invention and illustrated in 1, is virtuallylogarithmic.

While the present-invention has been described in connection with anelectron multiplier amplifier havingr va photocatbode arranged toprovide an electronic input to the secondary emission controlling meansin Vresponse: to light imningincr the photocathode. it is to be1'nd^rstood that the inventionis not limited to this arrangement, Vsince`any type of electronic input to lan electron -multiplier tube cold besubstituted .for

the photoelectric input. For example, the -l^c tronic input to thesecondarv emission controlling means .might be of a tberrpionic type. orva rious other tyres loi" electronic input may be emploved with` thepresent invention. While the present invention is described as employingsecondary emission electrodes, it is not limited to such. anarrangement, since any type of secondary emission controlling means maybe employed.

For example, vmagnetic electron focusing and/or secondarv emissioncontrolling means maybe employed. Vsingle or in conjunction withelectrostatic secondary ,electron emission controlling and/or'focusingmeans.

The referencing `means mayalso be modified Within wide limits Withoutdeparting Vfrom the spirit of the present invention. Any means Vforproviding intermittent iixed reference modulation of system input andsimultaneous inverse feedback to the amplifier for causing themodiiication of the-ampliner output to a xedstandardbearing a certainpreselected relationship to the 'iixed selected Vreference input may beemployed. Numerous types of inverse feedback means may he employed. Themeans for .modifying thelight input may also be modiiied .within Widelimits.

Those skilled inthe art will, from .the description given, understandthat both a constant density stripand a vane may be used in modulatingthe light supply to the phototube simultaneously, or sequentially. Y

The 'inverse Vfeedback may be applied to the secondary emissioncontrolling means rather than'to the electronic input to the electronmultiplier tube if desired. Thisis more particularly described,illustrated andclaimed in my copending application, SerialNo. 55,034,filed concurrently herewith, which is now U. S. Patent No. 2,534,668,granted December 19. 1,950.

The examples described ,and illustrated herein are 'exemplary only,andare not intended to limit lthe-scope of the present invention, whichis to be interpreted inthe light of the appended claims only.

Iclaim: 1. A method of compensating for changes Yin responsecharacteristics of a multiplier phototube and vampliiier controllingsaid multiplier phototube during a scanning operation, which comprises:periodically interrupting the control,

lil

of said multiplier phototube by said amplifier and simultaneouslysubjecting the multiplier `photo'tube both to scanning light modulated"by a standard density and to a predetermined potential and adjustingthe intensity of the scanning light -in accordance With the response of`the phototube and amplier While under the'inuence of vsaid light andpredetermined potential.

2. A'method of compensating for changes in response characteristics `ofva multiplier phototube in .an operating circuit of a scanning .devicewhich comprises: periodically interrupting va scanning operationandsubjecting the multiplier phototube to a .predetermined'potentiatanda light modulated by a standard density and aidjusting the' lightinaccordance with the response 'of-thephototube While under the 'influenceof .the predetermined Ypotential andk .modulated j light; andcontinuingthe scanning operation withjthe lightinits adjusted state.

`3. In a method of automatically .compensating for changesin responsecharacteristics oanam- ,.plirler including a .multiplier phototube.having Ya .plurality of secondary .emission electrodes, .the steps of:intermittently and frequently interrupt- -ingthe operation of anamplifier Vfor a periodpf Vshort durationV and subiecting the.multiplier phototuhe .to a 'predetermined .uniform potential and .alight .of `a yalue `expecteri to produce .a selected, fixed responsefrom thephototube; .and adjusting thelight input to Athe phototuloedur.- ingsaid period in a` negative feedback mannerin accordance withdeviation Vcf 'thephototdbeeresponse, while under .said L.predeterminedpotential, Vfrom the selected response; vand.continuing the operation`0l" the amplifier with the lightinits adjusted state until -.the.succeeding .checking period. Y l

4.1m a method of .correcting forlchanges ,in .sensitivity .of -ascanning system, including .a scanning `light and .an amplifier provided.with a phototube, 1the steps .of -periodically and .intermittentlyinterrupting the scanning .operation and subjecting .the phototube .to.a predetermined, uniformpotential and .a .lightnia value Aexpected toproduce a `selected response from ithephototube; adjusting thelightinput .to the-.photocell in accordance with. deviatior-i-of .thephototube response, while under said Ypredetermined .potential., fromtheselected response; and continuing .the scanning .operation with.thelightin gitsad- .justedstate thereof: means for supplying light; aninverse .feedbackmeans` operably connected to ythe-phototube .andamplifier, said Yfeedback means being 4responsive Vto phototube output`to modify Jthe light to Ythe vphototube rin accordance -with the'output of the phototube; means for periodically and intermittentlyrestoring the modulation :of the Isystem input to a selected, fixedreference value; Tand means 'fori-rendering the inverse 4feedback meansresponsive -only during said intermittent checking periods, saidfeedback means being arranged to modifythe light input in a directiontending to return the output -signal-to a selected standard valuewhenever, during such checking periods, the -phototube output differsfrom a desired value. Y

6. A system of the characterstatedin claim 5 wherein the phototube is anelectron .multiplier phototube amplier .of the type including .-.a

if Pmioenive Cathode., an Output anode and a plurality of secondaryemission electrodes, and wherein the modulated system input is the lightinput to the photosensitive cathode of the phototube.

'7. A system of the characterstated in claim wherein the means forintermittently restoring the system input toa selectdvalue include meansfor applying an electric signal of a selected fixed reference valueacross the secondary emission electrodes of the electron multiplierphototube amplifier during the simultaneous periods when the inversefeedback means is responsive to deviation of the amplified phototubeoutput signal from the selected standard value, and when4 the lightinput to the phototube is modulated by a selected reference fixeddensity.

8. A system of the character stated in claim 5 wherein the means forintermittently restoring the modulation of the system input comprises amechanically driven scanning system arranged to carry an image to bescanned by the phototube, said image being provided with a strip ofconstant modulating effectiveness arranged to intermittently modify themodulation of the light input to the phototube to a selected fixedreference value during scanning of the image.

l9. A scanning system of the character stated in claim 5 wherein thephototube is a multiplier including a photosensitive cathode, an outputanode, and a plurality of secondary emission electrodes; and the meansfor intermittently restoring the modulation of the system input comprisea mechanically driven scanning drum, a strip of constant modulatingeffectiveness arranged to intermittently modify the modulation of thelight input to the phototube and switch means correlated with thescanning drum to intermittently apply an electric signal of desiredreference value across the secondary emission electrodes of thephototube simultaneously with the modulation of the light input to thephototube by the strip.

10. A scanning system of the character stated in claim 5 wherein theinverse feedback means include a reversible motor, variable rheostatmeans operably connected to the motor, a light source under the controlof the rheostat, reversing switch means for energizing the motor forrotation in either direction, and relay means responsive to deviation ofthe amplified phototube output for actuating the reversing switch means.

11. A scanning system of the character stated in claim 5 wherein theinverse `feedback means include a reversible motor, variable rheostatmeans operably connected to the motor, a light source under the controlof the rheostat, reversing switch means for energizing the motor forrotation in either direction, and relay means responsive to deviation ofthe amplified phototu'oe output for actuating the reversing switchmeans, the means for intermittently rendering the feedback meansresponsive including switch means arranged to energize and de-energizethe motor in timed relation.

12. A scanning system of the character stated in claim 5 wherein theinverse feedback means includes reversible motor means, light valvemeans operably connected to the motor and arranged to modify light inputto the phototube in accordance with the direction of rotation of themotor, reversing switch means for energizingthe motor for rotation ineither direction, and relay means responsive to deviation of th'ephototube output for actuating the reversing switchmeans.A 13. A systemof the character stated in claim 5 wherein the inverse feedback meanscomprises electric oscillation generating means; and including modulatormeans arranged to modulate the output oscillations produced by theelectric oscillation generator in accordance with the amplifieddeviation of phototube output current from a-fixed reference value; andscanning lamp means arranged to be effectively energized by the outputelectric oscillations and to supply light input to the phototube.

14. A system of the character stated in claim 5 wherein the inversefeedback means comprises electric oscillation generating means; andincluding modulator means arranged to modulate the output oscillationsproduced by the electric oscillation generator in accordance with theamplified deviation of phototube output current from a fixed referencevalue; and scanning lamp means arranged to be effectively energized bythe output electric oscillations and to supply light input to thephototube, the means for rendering the inverse feedback means responsiveto the phototube output including switch means arranged to connect anddisconnect the amplified phototube output to the modulator means in theinverse feedback means in timed relation to the scanning of an image andonly during checking periods when the modulation of the light input intothe phototube is of fixed reference value.

15. A system of the character stated in claim 13 wherein the electricoscillation Vgenerating means is of the square wave generating type.

16. In an electron multiplier amplifier system arranged to receive amodulated system input, the provision of: means for supplying electronicinput to the electron multiplier amplifier; and inverse feedback meanseffectively responsive to the electron multiplier output signal arrangedto modify the electronic input to the electron multiplierin a negativefeedback sense in accordance with the electron multiplier output signal,whereby sensitivity drift of the amplifier will be minimized.

1'7. A system of the character stated in claim 16 including: means forintermittently, alternately, effectively rendering the inverse feedbackmeans effectively responsive and non-responsive to the electronmultiplier output signal, whereby the electronic input to the electronmultiplier amplifier will be modified intermittently in a negativefeedback sense in accordance with the electron multiplier output signalduring the corresponding intermittent responsive periods.

18. In an electro-optical amplifier system arranged to receive amodulated system input rand including a phototube and an amplierconnected to the output thereof to produce an amplified phototube outputsignal, the provision of means for supplying light input to thephototube; inverse feedback means responsive to the amplified phototubeoutput signal arranged to modify the light input to the phototube in anegative feedback sense in accordance with the amplified phototubeoutput signal; and means for periodically and repetitively rendering theinverse feedback means effectively responsive fora period of shortduration to the amplified phototube output signal, whereby the lightinput to the phototube will be modified periodically duringl thecorresponding intermittent responsive periods and sensitivity drift ofthe system will be minimized.

19. A system of the character stated in claim 19 gummi;

I5 16 means for intermittentlyrestoring the UNITED? SFIMYISES PATENTSmodulationsof theesystem nputmo a selectedxed. Number Name Datereference value during the simultaneous, inter- 20.12.573 Long. Aug. 271935 mittenuperiods when the inverse feedback means 2153193'Morse"`.`""`"` Mag; 16 1939 is: responsive to the ampled PhototubeOutput 6 213472015 Wolosc-n-nmu Apr. 18-1 1944 Slanal 2,41-1f,44,o LeYPage Nov. 19, 1946 NORMAN R- GUNDERSON 2,412,423.. Rajcnman et a1. Dec.-10, 194e REFERENCES CITED The following references are of record in the10 fue ofthis patent:

